Hammermills have long been used for grinding or comminution of materials. Typically hammermills consist of a rotor mounted on a solid through rotor shaft inside a housing. A material inlet is generally located at the top of the housing with one or more material outlets located near the bottom of the housing. The rotor includes a solid through drive shaft and rows of hammers which are normally flat steel blades or bars. A steel rod or pin pivotably connects the hammer to the rotor. The rotor is mounted inside a typically teardrop shaped enclosure, commonly known as a grinding or working chamber, which is comprised of a cutting plate mounted on either side of the material inlet for reversible hammermills. Reversible hammermills are capable of rotation in either direction, a feature which provides for increased life for the hammers, cutting plates and screen plates. The known cutting plates are comprised of a upper linear section connected with a convex radiused section and do not allow particles to escape.
Downstream of the cutting plate, the interior of the working chamber is defined by curved screen plates. The screen opening diameter is selected to match the desired particle size. Generally, material at or below an intended size limit exit the chamber through the screens while material above the size limit continue to be reduced by the rotating hammers.
Current hammermill rotor designs consist of a solid through rotor shaft which supports a number of cylindrical head disks. The head disks are keyed to the shaft and are spaced along the shaft with ring type spacers, often squeeze collars or the equivalent are employed. The head disks and spacers are held together on the rotor shaft by using bearing locknuts which are positioned on the threaded ends of the rotor shaft. These nuts are then tightened to take the clearance out between the disks and the spacers.
The disks structurally support a number of hammer pins radially around the solid rotor shaft. The swinging hammers are mounted on the hammer pins. The disks structurally support the hammer pins from the centrifugal forces generated by the rotation of the rotor which typically rotates over a range of 1200 to 3600 rpm. The disks also transmit the torque from the rotor shaft to the hammer pins; required to power the hammers through their impact against the product being processed in the hammermill.
In operation, the material to be reduced is fed into the material inlet and is directed toward the rotating hammers. The material is initially impacted by the hammers, which may cause some material reduction. The material is then flung from the hammer face against the cutting plates resulting in a primary reduction of material. After the material impacts the cutting plate, from which there is typically no outlet, the material is either flung back toward the rotating hammers or continues downstream between the hammer tip and the cutting plate until the screen plates are reached.
Ultimately, the particles encounter the openings of the screen plates. Here, the particles that are small enough begin to exit through the screen openings. The remaining particles impact the leading edge of the screen openings and are deflected up into the hammers' path. The rotating hammers continue to pulverize the material downstream of the cutting plate, moving it along the surface of the screens which define the circumference of the working chamber, causing gradual diminution of the material. Ultimately, the material is ground finely enough to permit it to flow out through the screens.
Pulverizers work generally in the same manner as hammermills except that non-swinging knives are used in place of the swinging hammers. This configuration allows for a finer particle size than the typical hammermill, but results in greater wear damage to the rigid knives.
While the hammermill and pulverizer designs as described above have been generally accepted and is widely used, there is a constant need and desire to achieve a finer particle size without the associated wear and stress encountered in known designs.
The present invention accomplishes these goals.